This invention relates to a drum brake having a dual mode of operation. In a service mode, a hydraulic actuator moves friction members on brake shoes into engagement with a drum to effect a first brake application and in a parking brake mode, a mechanically operated lever pivots on an end member of an adjustable strut to move the friction members into engagement with the drum and effect a second brake application.
Duo-mode drum brakes of a type disclosed in U.S. Pat. Nos. 4,249,646 and 4,364,456 are frequently used in combination on vehicles having front wheel disc brakes. For parking brake functions, the disc brake is mechanically actuated by applying a force through a lever arm that is carried on a web of a brake shoe. The lever arm has a first end which is pivotally attached to the web and a second end which is linked to a cable located adjacent a pivot block. The cable typically being attached to the second end in a manner as disclosed in U.S. Pat. No. 5,706,914. The mechanical actuation of this type drum brake is capable of providing sufficient braking force for parking and is adequate for most applications, however, the cost associated with manufacturing has resulted in customers requesting an alternate for implementing the function of a parking brake. In-addition when this type brake is used on vehicles identified as sport-utility models it is desirous to have all components as far away from the ground as possible to avoid the engagement with sand, rocks, stumps, and non-level ground.
In an effort to develop a robust brake for the rear of a front wheel drive vehicle it has been proposed to use disc brakes on all of the wheels and provide for a parking brake function through a drum brake-disc brake, as disclosed in U.S. Pat. Nos. 5,180,037 and 5,529,149 and commonly referred to as drum-in-hat. The drum brake in this structure is actuated through a scissors arrangement that responds to mechanical input. In an effort to simplify a parking brake structure, a lever arrangement, of the type disclosed in U.S. Pat. Nos. 5,311,971 and 6,206,148, extends through a backing plate and responds to a linear force to provide a mechanical force for activating a parking brake. This structural arrangement performs in an adequate manner but the cost and weight has resulted in some manufacturers looking for a low cost and lightweight brake. The structure disclosed in U.S. Pat. Nos. 5,311,793 and 6,234,281 simplified the parking brake function but would require a manual adjustment to maintain a desired running clearance during the operational life of the friction material of the brakes.
On review of the use and performance required for braking a front wheel driven vehicle or a rear wheel drive on an all wheel drive vehicle, it was determined that a drum brake could meet the operational requirements and the parking brake function could be achieved through a dual use of an adjustable strut assembly for a drum brake.
In the present invention, a drum brake assembly has first and second brake shoes that are retained on a backing plate by first and second pins. Each of the first and second brake shoes has a first end that engage a hydraulic actuator assembly and a second end aligned on and urged toward an anchor block on the backing plate by a spring arrangement. The first ends are spaced apart by the adjustable strut assembly that has an extendable shaft with a first end separated from a second end by a rotatable stem. A first slot in the first end engages a first web of the first brake shoe adjacent its first end to prevent the first end from rotating. Similarly, the second end has a second slot that engages a second web of the second brake shoe adjacent its first end to prevent the second end from rotating. The rotatable stem has a first cylindrical surface retained in a bore in the first end, a second cylindrical surface that engages the first end and a third cylindrical surface with first threads thereon which mate with second threads in its second end. The rotatable stem is selectively adjusted by a ratcheting action of an arm carried on the first web such that its first end moves toward the first web and its second end moves toward the second web to maintain a predetermined running clearance between a drum and a first friction pad on the first brake shoe and a second friction pad on the second brake shoe. The hydraulic actuator assembly is responsive to a first input to respectively move the first friction pad and the second friction pad into engagement with the drum to effect a first brake application. A mechanical actuator is connected to the strut mechanism to provide a second input to respectively move the first and second friction pads into engagement with the drum to effect a second brake application. The mechanical actuator is characterized in that the second end of said extendable shaft is defined by a flat base having a substantially first oval shape that transitions into a cylindrical projection. The second threads that mate with the first threads on the rotatable stem are located in the cylindrical projection and the second slot is located in the flat base along an axis of the second threads. The flat base has a first hole located adjacent the second slot for receiving a pin which extends through a second hole in a second flat base that forms a first end of a lever. The first end of the lever is offset from its second end such that the second end is substantially in a plane parallel with the rotatable stem when a ledge on the second flat base engages the second web. The pin that extends through the first hole and the second hole maintains the first flat base and the second flat base in substantially parallel planes. Linkage consisting of a cable extends through the backing plated is attached to the second end of the lever and on receipt of a linear force applied to the second end along a plane perpendicular to the rotatable stem causes the second flat base to pivot on the pin and transfer an axial force from the ledge into the second web substantially along the axis of the cylindrical projection to effect a parking brake application.
An advantage of the present invention resides in a mechanical actuator with an extendable strut having a first end member with a substantially oval shape which is aligned with an end of a lever having a semi-oval shape by a pin to locate a ledge on the semi-oval shape with a web of a brake shoe such that an input force applied to the lever causes the first end to pivot on the pin and transmit a axial force through the extendable strut to effect a parking brake function of a vehicle.
A further advantage of this invention is to provide a drum brake with a dual mode of actuation whereby a service brake is converted to a parking brake by substituting a manual input through a lever carried on a adjustable strut which maintains a desired running clearance.